Stabilizers, carbodiimides

Substituent effects on the stability of carbodiimides show that electron negative substituents, such as F, Cl, OH and NH2 destabilize carbodiimides, while electropositive substituents increase the stability of carbodiimides. However, the electronegative substituent NO2 stabilizes carbodiimides by a tt-acceptor complex. ... 

After the ring-enlargement reaction has been accomplished, the iminophos-phorane group can be employed in a subsequent step with phenyl isocyanate, resonance-stabilized carbodiimides (281) are obtained, which proved to be versatile starting materials for many heterocyclization reactions leading to novel 6 7, 6 8, and 5 6 7 combinations of heterocondensed pyrimidines (282)153 (Scheme 77). 

Rafat M, Li F, Fagerhohn P et al (2008) PEG-stabilized carbodiimide crosslinked col-lagen-chitosan hydrogels for corneal tissue engineering. Biomaterials 29 3960-3972... 

A first terminal imido complex of nickel (121) was prepared according to Equation (3).468 The synthesis goes via the Ni11 amido compound (122) and uses the steric bulk of the arylimido group for stabilization. The Ni11 center in (121) is planar and three-coordinate. Reaction of (121) with CO or benzyl isocyanide leads to formal nitrene transfer with formation of (124) and (125), respectively. Further reaction with CO liberates the isocyanate and carbodiimide (Equation (4)). 69... 

Gilles, M.A., Hudson, A.Q., and Borders, C.L. (1990) Stability of water-soluble carbodiimides in aqueous solution. Anal. Biochem. 184, 244-248. 

This material is commonly found as a degradation product from hydrolysis of VX stabilizer Dicyclohexyl carbodiimide (C01-C049). 

Brown, D.F., A.J. Policastro, W.E. Dunn, R. A. Carhart, M. A. Lazaro, W. A. Freeman, and M. Krumpolc. Development of the Table of Initial Isolation and Protective Action Distances for the 2000 Emergency Response Guidebook, Argonne National Laboratory Report No. ANL/DIS-00-1. October 2000. Buckles, L.C. and S.M. Lewis. "S-(2-Diisopropylamino-ethyl)0-ethyl methylphosphonothioate Stabilized with Soluble Carbodiimides," United States Patent 4012464, September 24,1965. Centers for Disease Control and Prevention. "Case Definition Nerve Agents or Organophosphates." March 15,2005. 

Full -polarization in diradicals can give rise to zwitterionic products. First examples were studied in detail by Carpenter and coworker who investigated solvent effects on rates and product distribution in Myers-Saito cyclizations.64 Polar solvents and substitution patterns that stabilize either positive or negative charges (or both) favor the zwitterionic products. For example, the presence of a dimethylamino group leads to stabilization of cations and isolation of pyrrolo-quinolines, rather than pyrido-indoles from eneyne-carbodiimides, as reported by Wang and coworkers (Scheme 14).65... 

At room temperature aliphatic and aromatic carbodiimides are liquid or solid. They usually can be purified by distillation under reduced pressure or by recrystallization to yield neutral products. On prolonged standing they polymerize to yield basic products. The liquid carbodiimides are less stable than the solid ones. The stability of carbodiimides increases with the degree of branching of the alkyl substituents attached to the nitrogen atom RCH2 < R2CH < R3C. For example, diethylcarbodiimide [14, 21] polymerizes in a few days, whereas diisopropyl- and dicyclohexylcarbodiimides [14] are stable for several months. Unsaturated substituents also cause a marked decrease in stability... 

Carbodiimide-mediated amide bond formation effectively occurs between pH 4.5 and 7.5. Buffer systems using MES or phosphate may be used to stabilize the pH during the course of the reaction. For additional information on specific carbodiimides used in bioconjugate chemistry, see Chapter 3, Section 1. 

Depending on their stability, activated esters can be generated and stored as pure reagents on solid supports. The by-products of carbodiimides... 

Hydrolysis of stabilizer N-N -diisopropyl carbodiimide, 17 2-3 Simplified scheme for the hydrolysis of VX, 17 2-4 Major hydrolysis pathways for mustard, 18... 

Carbodiimides have been found to stabilize the polyester polyurethanes by blocking the carboxyl group formed when the chain has been broken. This helps prevent further autocatalytic degradation. The action is most pronounced when amines are used as the curative in caprolactone polyurethanes. 

With regard to reactive flame-retardants, two routes can be followed to improve thermal stability and fire behavior of PU foams use of brominated or phosphorus-containing polyol or, for rigid foams, the introduction inside polymer backbone of more thermally stable structure than urethane, mainly isocyanurate, but also uretidione rings or carbodiimide.19... 

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